Wet Electrolytic Capacitor for an Implantable Medical Device

ABSTRACT

A wet electrolytic capacitor containing a cathode, fluidic working electrolyte, and planar anode formed from an anodically oxidized sintered porous pellet is provided. The pellet may be formed from a pressed valve metal powder, which in turn, is formed by reacting an oxide of a valve metal compound (e.g., tantalum pentoxide) with a reducing agent that contains a metal having an oxidation state of 2 or more (e.g., magnesium). Through the use of such a powder, the present inventors have discovered that higher capacitance levels can be achieved than previously thought possible for the high voltage capacitors employed in implantable medical devices.

BACKGROUND OF THE INVENTION

High voltage electrolytic capacitors are often employed in implantablemedical devices. These capacitors are required to have a high energydensity because it is desirable to minimize the overall size of theimplanted device. This is particularly true of an implantablecardioverter defibrillator (“ICD”), also referred to as an implantabledefibrillator, because the high voltage capacitors used to deliver thedefibrillation pulse can occupy as much as one third of the ICD volume.ICDs typically use two to four electrolytic capacitors in series toachieve the desired high voltage for shock delivery. Typically, metalfoils (e.g., aluminum foil) are used in the electrolytic capacitor dueto their small size. Because the electrostatic capacitance of thecapacitor is proportional to its electrode area, the surface of themetallic foil may be, prior to the formation of the dielectric film,roughened or subjected to a chemical conversion to increase itseffective area. This step of roughening the surface of the metallic foilis called etching. Etching is normally carried out either by the method(chemical etching) of conducting immersion into a solution ofhydrochloric acid or by the method (electrochemical etching) of carryingout electrolysis in an aqueous solution of hydrochloric acid. Thecapacitance of the electrolytic capacitor is determined by the extent ofroughing (the surface area) of the anode foil and the thickness and thedielectric constant of the oxide film.

Due to the limited surface area that may be provided by etching metallicfoils, attempts have also been made to employ porous sintered pellets inwet electrolytic capacitors—i.e., “wet tantalum” capacitors. A tantalumpellet, for instance, may be formed by compressing a powder under highpressure and sintering at high temperature to form a sponge-likestructure, which is very strong and dense but also highly porous. As aresult of the high voltages encountered in medical devices, however, lowspecific charge powders must generally be employed. Namely, if thespecific charge is too high, relatively thin sinter necks tend to formbetween adjacent particles, which can cause the dielectric layer in thevicinity of these necks to fail at high voltages.

As such, a need currently exists for an improved wet electrolyticcapacitor for use in implantable medical devices, such asdefibrillators.

SUMMARY OF THE INVENTION

In accordance with one embodiment of the present invention, a wetelectrolytic capacitor is disclosed that comprises a planar anode,cathode, and fluidic working electrolyte in communication with the anodeand cathode. The anode comprises an anodically oxidized pellet formedfrom a pressed and sintered valve metal powder. The valve metal powderis formed by reacting an oxide of a valve metal compound with a reducingagent that contains a metal having an oxidation state of 2 or more. Thecathode comprises a metal substrate coated with a conductive coating.

In accordance with another embodiment of the present invention, a wetelectrolytic capacitor is disclosed that comprises a planar anode,cathode, and fluidic working electrolyte in communication with the anodeand cathode. The anode comprises an anodically oxidized pellet formedfrom a pressed and sintered tantalum powder. The powder is nodular orangular and has a specific charge of about 15,000 μF*V/g or more. Thecathode comprises a metal substrate coated with a conductive coating.

In accordance with yet another embodiment of the present invention, amethod for forming a wet electrolytic capacitor is disclosed thatcomprises pressing a tantalum powder into the form of a pellet, whereinthe powder is formed by reacting tantalum pentoxide with a reducingagent that contains magnesium, calcium, strontium, barium, cesium,aluminum, or a combination thereof; sintering the pellet; anodicallyoxidizing the sintered pellet to form a dielectric layer that overliesthe anode; and positioning the anode and a fluidic working electrolytewithin a casing.

Other features and aspects of the present invention are set forth ingreater detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of the present invention, including thebest mode thereof, directed to one of ordinary skill in the art, is setforth more particularly in the remainder of the specification, whichmakes reference to the appended figures in which:

FIG. 1 is a perspective view of one embodiment of the wet electrolyticcapacitor of the present invention;

FIG. 2 is a top view of embodiment of an anode that may be employed inthe capacitor of the present invention;

FIG. 3 is a frontal view of the anode of FIG. 2; and

FIG. 4 is a perspective view illustrating the assembly of the anode ofFIG. 2 with casing components to form the capacitor shown in FIG. 1.

Repeat use of references characters in the present specification anddrawings is intended to represent same or analogous features or elementsof the invention.

DETAILED DESCRIPTION OF REPRESENTATIVE EMBODIMENTS

It is to be understood by one of ordinary skill in the art that thepresent discussion is a description of exemplary embodiments only, andis not intended as limiting the broader aspects of the presentinvention, which broader aspects are embodied in the exemplaryconstruction.

Generally speaking, the present invention is directed to a wetelectrolytic capacitor that contains a cathode, fluidic workingelectrolyte, and planar anode formed from an anodically oxidizedsintered porous pellet. The pellet may be formed from a pressed valvemetal powder, which in turn, is formed by reacting an oxide of a valvemetal compound (e.g., tantalum pentoxide) with a reducing agent thatcontains a metal having an oxidation state of 2 or more. Examples ofsuch metals may include, for instance, alkaline earth metals (e.g.,magnesium, calcium, strontium, barium, cesium, etc.), aluminum, and soforth. Through the use of such a powder, the present inventors havediscovered that higher capacitance levels can be achieved thanpreviously thought possible for the high voltage capacitors employed inimplantable medical devices.

Various embodiments of the present invention will now be described inmore detail.

I. Anode

The anode is formed from a valve metal powder that contains a valvemetal (i.e., metal that is capable of oxidation) or valve metal-basedcompound, such as tantalum, niobium, aluminum, hafnium, titanium, alloysthereof, oxides thereof, nitrides thereof, and so forth. For example,the valve metal powder may contain an electrically conductive oxide ofniobium, such as niobium oxide having an atomic ratio of niobium tooxygen of 1:1.0±1.0, in some embodiments 1:1.0±0.3, in some embodiments1:1.0±0.1, and in some embodiments, 1:1.0±0.05. The niobium oxide may beNbO_(0.7), NbO_(1.0), NbO_(1.1), and NbO₂.

As noted above, the valve metal powder may be formed by reacting anoxide of a valve metal compound with a reducing agent that contains ametal having a relatively high oxidation state (e.g., magnesium). Thevalve metal oxide is typically a tantalum and/or niobium oxide capableof being reduced, such as Ta₂O_(x) (x≦5) (e.g., Ta₂O₅) or Nb₂O_(x) (x≦5)(e.g., Nb₂O₅). The reducing agent may be provided in a gaseous, liquid,or solid state, and may also be in the form of the metal, as well asalloys or salts thereof. In one embodiment, for instance, a halide salt(e.g., chloride, fluoride, etc.) may be employed. If desired, othercomponents may also be added before, during, or after the reaction, suchas dopants, alkali metals, etc. The reduction of the oxide is typicallycarried out at a temperature of from about 400° C. to about 1200° C.,and in some embodiments, from about 600° C. to about 1000° C., for about20 to about 300 minutes. Heating may be carried out in a reactor underan inert atmosphere (e.g., argon or nitrogen atmosphere) so that amolten bath is formed. Suitable reactors may include, for instance,vertical tube furnaces, rotary kilns, fluid bed furnaces, multiplehearth furnaces, self-propagation high-temperature synthesis reactors,etc. The reactor may be maintained under inert gas until that the massin the reaction vessel is cooled to ambient temperature. Additionaldetails of such a reduction reaction may be described in U.S. PatentPublication Nos. 2003/0110890 to He, et al. and 2004/0163491 toShekhter, et al.

After the reduction, the product may be cooled, crushed, and washed toremove excess impurities or reactants. The washing solution may include,for instance, a mineral acid and water. If desired, the powder may besubjected to additional treatment to remove any tantalates/niobates(e.g., magnesium tantalate) that may have formed during the reaction. Inone embodiment, for instance, one technique for removing thetantalates/niobates involves heating the powder under vacuum at atemperature of from about 1100° C. to about 1400° C. for about 15minutes to about 6 hours. Likewise, another technique for removing thetantalates/niobates involves heating the powder at a temperature of fromabout 800° C. to about 1300° C. in the presence of a getter material,such as magnesium, calcium and/or aluminum, for about 15 minutes toabout 6 hours. Such techniques may be described in more detail in U.S.Pat. No. 7,431,751 to Shekhter, et al. Although not required, the powdermay be subjected to additional refining steps as is known in the art,such as doping, deoxidizing, etc.

Regardless of the particular steps employed, the resulting powder has avariety of beneficial properties. The powder may, for example, be afree-flowing, finely divided powder that contains primary particleshaving a three-dimensional shape, such as a nodular or angular shape.Such particles are not generally flat and thus have a relatively low“aspect ratio”, which is the average diameter or width of the particlesdivided by the average thickness (“D/T”). For example, the aspect ratioof the particles may be about 4 or less, in some embodiments about 3 orless, and in some embodiments, from about 1 to about 2. The powder mayalso have a relatively high specific surface area, such as about 1square meter per gram (“m²/g”) or more, in some embodiments about 2 m²/gor more, and in some embodiments, from about 4 to about 30 m²/g. Theterm “specific surface area” generally refers to surface area asdetermined by the physical gas adsorption (B.E.T.) method of Bruanauer,Emmet, and Teller, Journal of American Chemical Society, Vol. 60, 1938,p. 309, with nitrogen as the adsorption gas. The test may be conductedwith a MONOSORB® Specific Surface Area Analyzer available fromQUANTACHROME Corporation, Syosset, N.Y., which measures the quantity ofadsorbate nitrogen gas adsorbed on a solid surface by sensing the changein thermal conductivity of a flowing mixture of adsorbate and inertcarrier gas (e.g., helium).

The primary particles of the powder may also have a median size (D50) offrom about 5 to about 1000 nanometers, and in some embodiments, fromabout 10 to about 500 nanometers, such as using a laser particle sizedistribution analyzer made by BECKMAN COULTER Corporation (e.g.,LS-230), optionally after subjecting the particles to an ultrasonic wavevibration of 70 seconds. Due to its high surface area and low particlesize, the powder may have a high specific charge, such as greater thanabout 15,000 microFarads*Volts per gram (“μF*V/g”), in some embodimentsfrom about 18,000 to about 80,000 μF*V/g, and in some embodiments, fromabout 20,000 to about 45,000 μF*V/g. As is known in the art, thespecific charge may be determined by multiplying capacitance by theanodizing voltage employed, and then dividing this product by the weightof the anodized electrode body. Despite the use of such high specificcharge powders with three-dimensional particles, the present inventorshave nevertheless discovered that the ability to achieve high voltagescan be achieved through the manner in which the powder is formed. Moreparticularly, it is believed that the particular reduction processemployed can achieve “sinter necks” between adjacent agglomeratedparticles that are relatively large in size. Sinter necks are the smallcross-sectional area of the electrical path within the metal structure.Typically, the sinter necks have a size of about 200 nanometers or more,in some embodiments about 250 nanometers or more, and in someembodiments, from about 300 to about 800 nanometers. Because the necksare relatively large in size, the dielectric layer in the vicinity ofthe neck is more likely not to fail at high forming voltages.

The powder (as well as the anode) may also have a relatively low alkalimetal, carbon, and oxygen content. For example, the powder may have nomore than about 50 ppm carbon or alkali metals, and in some embodiments,no more than about 10 ppm carbon or alkali metals. Likewise, the powdermay have no more than about 0.15 ppm/μC/g oxygen, and in someembodiments, no more than about 0.10 ppm/μC/g oxygen. Oxygen content maybe measured by LECO Oxygen Analyzer and includes oxygen in natural oxideon the tantalum surface and bulk oxygen in the tantalum particles. Bulkoxygen content is controlled by period of crystalline lattice oftantalum, which is increasing linearly with increasing oxygen content intantalum until the solubility limit is achieved. This method wasdescribed in “Critical Oxygen Content In Porous Anodes Of Solid TantalumCapacitors”, Pozdeev-Freeman et al., Journal of Materials Science:Materials In Electronics 9, (1998) 309-311 wherein X-ray diffractionanalysis (XRDA) was employed to measure period of crystalline lattice oftantalum. Oxygen in sintered tantalum anodes may be limited to thinnatural surface oxide, while the bulk of tantalum is practically free ofoxygen.

To facilitate the construction of the anode, certain additionalcomponents may also be included in the powder. For example, the powdermay be optionally mixed with a binder and/or lubricant to ensure thatthe particles adequately adhere to each other when compacted or pressedto form the pellet. Suitable binders may include, for instance,poly(vinyl butyral); poly(vinyl acetate); poly(vinyl alcohol);poly(vinyl pyrollidone); cellulosic polymers, such ascarboxymethylcellulose, methyl cellulose, ethyl cellulose, hydroxyethylcellulose, and methylhydroxyethyl cellulose; atactic polypropylene,polyethylene; polyethylene glycol (e.g., Carbowax from Dow ChemicalCo.); polystyrene, poly(butadiene/styrene); polyamides, polyimides, andpolyacrylamides, high molecular weight polyethers; copolymers ofethylene oxide and propylene oxide; fluoropolymers, such aspolytetrafluoroethylene, polyvinylidene fluoride, and fluoro-olefincopolymers; acrylic polymers, such as sodium polyacrylate, poly(loweralkyl acrylates), poly(lower alkyl methacrylates) and copolymers oflower alkyl acrylates and methacrylates; and fatty acids and waxes, suchas stearic and other soapy fatty acids, vegetable wax, microwaxes(purified paraffins), etc. The binder may be dissolved and dispersed ina solvent. Exemplary solvents may include water, alcohols, and so forth.When utilized, the percentage of binders and/or lubricants may vary fromabout 0.1% to about 8% by weight of the total mass. It should beunderstood, however, that binders and/or lubricants are not necessarilyrequired in the present invention.

The resulting powder may be compacted to form a pellet using anyconventional powder press device. For example, a press mold may beemployed that is a single station compaction press containing a die andone or multiple punches. Alternatively, anvil-type compaction pressmolds may be used that use only a die and single lower punch. Singlestation compaction press molds are available in several basic types,such as cam, toggle/knuckle and eccentric/crank presses with varyingcapabilities, such as single action, double action, floating die,movable platen, opposed ram, screw, impact, hot pressing, coining orsizing. The powder may be compacted around an anode lead wire. The wiremay be formed from any electrically conductive material, such astantalum, niobium, aluminum, hafnium, titanium, etc., as well aselectrically conductive oxides and/or nitrides of thereof.

Any binder/lubricant may be removed after pressing by heating the pelletunder vacuum at a certain temperature (e.g., from about 150° C. to about500° C.) for several minutes. Alternatively, the binder/lubricant mayalso be removed by contacting the pellet with an aqueous solution, suchas described in U.S. Pat. No. 6,197,252 to Bishop, et al. Thereafter,the pellet is sintered to form a porous, integral mass. The pellet istypically sintered at a temperature of from about 800° C. to about 2000°C., in some embodiments from about 1200° C. to about 1800° C., and insome embodiments, from about 1500° C. to about 1700° C., for a time offrom about 5 minutes to about 100 minutes, and in some embodiments, fromabout 8 minutes to about 15 minutes. This may occur in one or moresteps. If desired, sintering may occur in an atmosphere that limits thetransfer of oxygen atoms to the anode. For example, sintering may occurin a reducing atmosphere, such as in a vacuum, inert gas, hydrogen, etc.The reducing atmosphere may be at a pressure of from about 10 Torr toabout 2000 Torr, in some embodiments from about 100 Torr to about 1000Torr, and in some embodiments, from about 100 Torr to about 930 Torr.Mixtures of hydrogen and other gases (e.g., argon or nitrogen) may alsobe employed.

Upon sintering, the pellet shrinks due to the growth of metallurgicalbonds between the particles. Because shrinkage generally increases thedensity of the pellet, lower press densities (“green”) may be employedto still achieve the desired target density. For example, the targetdensity of the pellet after sintering is typically from about 5 to about8 grams per cubic centimeter. As a result of the shrinking phenomenon,however, the pellet need not be pressed to such high densities, but mayinstead be pressed to densities of less than about 6.0 grams per cubiccentimeter, and in some embodiments, from about 4.5 to about 5.5 gramsper cubic centimeter. Among other things, the ability to employ lowergreen densities may provide significant cost savings and increaseprocessing efficiency.

Due to the thin nature of the planar anode, it is sometimes desirable tocontrol the manner in which the anode wire is inserted to limit theextent that stresses applied during manufacturing will cause the wire topull out of the anode. For example, in one embodiment, at least aportion of the wire within the anode is bent at an angle relative to thelongitudinal axis of the wire. This “bend” reduces the ease to which thewire can be pulled out in the longitudinal direction after the anode ispressed and sintered. Referring to FIGS. 2-3, for example, oneembodiment of a planar anode 200 is shown that contains an anode wire220. The anode wire contains a first portion 221 that extends in alongitudinal direction (“y” direction) from the anode 200. Within thebody of the anode, the wire 200 also contains a second portion 222 thatis bent at an angle “a” relative to the first portion 221. The angle “a”is typically from about 40° to about 120°, in some embodiments fromabout 60° to about 110°, and in some embodiments, from about 80° toabout 100° (e.g., about 90°). Such a bent configuration may be achievedin a variety of different ways. For example, in one embodiment, a pressmold may be partially filled with the powder, and then a “pre-bent”anode wire may be inserted into the press mold. Thereafter, the mold maybe filled with powder and the entire assembly compressed into a pellet.

In addition to its geometric configuration, the extent to which theanode wire is inserted into the anode may also be controlled to helpminimize the likelihood of withdrawal during manufacturing. That is, thewire is less likely to be pulled out of the anode the farther it isinserted. Of course, too great of a wire insertion can alter theuniformity of the press density, which can impact the resultingelectrical performance of the anode. In this regard, the presentinventors have discovered that the ratio of the length of the anode inwhich the wire is inserted to the entire length of the anode istypically from about 0.1 to about 0.6, and in some embodiments, fromabout 0.2 to about 0.5. In FIG. 2, for example, the length “L₁”represents the length of the anode 200 in which the anode wire 220 isinserted, while the length “L” represents the entire length of the anode200. In certain cases, the length “L” of the anode 200 may range fromabout 1 to about 80 millimeters, in some embodiments from about 10 toabout 60 millimeters, and in some embodiments, from about 20 to about 50millimeters. Likewise, the length “L₁” may be from about 1 to about 40millimeters, in some embodiments, from about 2 to about 20 millimeters,and in some embodiments, from about 5 to about 15 millimeters. The width“W” of the anode may also be from about 0.05 to about 40 millimeters, insome embodiments, from about 0.5 to about 35 millimeters, and in someembodiments, from about 2 to about 25 millimeters.

The thickness of the planar anode is generally small to improve theelectrical performance and volumetric efficiency of the resultingcapacitor. In FIG. 3, for example, the thickness of a planar anode 200is represented by the dimension “H.” Typically, the thickness of theanode is about 5 millimeters or less, in some embodiments, from about0.05 to about 4 millimeters, and in some embodiments, from about 0.1 toabout 3.5 millimeters. The ratio of the length of the anode to thethickness of the anode is from about 5 to about 50, in some embodimentsfrom about 6 to about 30, and in some embodiments, from about 7 to about20. Although shown as a “D-shape” in FIG. 2, it should also beunderstood that the anode may possess any other desired shape, such assquare, rectangle, circle, oval, triangle, etc. Polygonal shapes havingmore than four (4) edges (e.g., hexagon, octagon, heptagon, pentagon,etc.) are particularly desired due to their relatively high surfacearea.

The anode also contains a dielectric formed by anodically oxidizing(“anodizing”) the sintered anode so that a dielectric layer is formedover and/or within the anode. For example, a tantalum (Ta) anode may beanodized to tantalum pentoxide (Ta₂O₅). Typically, anodization isperformed by initially applying a solution to the anode, such as bydipping anode into the electrolyte. Aqueous solvents (e.g., water)and/or non-aqueous solvents (e.g., ethylene glycol) may be employed. Toenhance conductivity, a compound may be employed that is capable ofdissociating in the solvent to form ions. Examples of such compoundsinclude, for instance, acids, such as described below with respect tothe electrolyte. For example, an acid (e.g., phosphoric acid) mayconstitute from about 0.01 wt. % to about 5 wt. %, in some embodimentsfrom about 0.05 wt. % to about 0.8 wt. %, and in some embodiments, fromabout 0.1 wt. % to about 0.5 wt. % of the anodizing solution. Ifdesired, blends of acids may also be employed.

A current is passed through the anodizing solution to form thedielectric layer. The value of the formation voltage manages thethickness of the dielectric layer. For example, the power supply may beinitially set up at a galvanostatic mode until the required voltage isreached. Thereafter, the power supply may be switched to apotentiostatic mode to ensure that the desired dielectric thickness isformed over the entire surface of the anode. Of course, other knownmethods may also be employed, such as pulse or step potentiostaticmethods. The voltage at which anodic oxidation occurs is typically highto achieve a capacitor capable of operating at a high voltage range.That is, the voltage is typically from about 100 volts to about 300volts, in some embodiments from about 170 volts to about 280 volts, andin some embodiments, from about 200 volts to about 250 volts. Thetemperature of the anodizing solution may range from about 10° C. toabout 200° C., in some embodiments from about 20° C. to about 150° C.,and in some embodiments, from about 30° C. to about 90° C. The resultingdielectric layer may be formed on a surface of the anode and within itspores. When employed, the specific nature of the powder may allow theresulting anode to achieve a high specific charge even at the highformation voltages often employed in the present invention.

II. Working Electrolyte

The working electrolyte may be in electrical communication with thecathode and anode. The electrolyte is a fluid that may be impregnatedwithin the anode, or it may be added to the capacitor at a later stageof production. The fluid electrolyte generally uniformly wets thedielectric on the anode. Various suitable electrolytes are described inU.S. Pat. Nos. 5,369,547 and 6,594,140 to Evans, et al. Typically, theelectrolyte is ionically conductive in that has an electricalconductivity of from about 0.1 to about 20 Siemens per centimeter(“S/cm”), in some embodiments from about 0.2 to about 15 S/cm, and insome embodiments, from about 0.5 to about 10 S/cm, determined at atemperature of about 23° C. using any known electric conductivity meter(e.g., Oakton Con Series 11). The fluid electrolyte is generally in theform of a liquid, such as a solution (e.g., aqueous or non-aqueous),colloidal suspension, gel, etc. For example, the electrolyte may be anaqueous solution of an acid (e.g., sulfuric acid, phosphoric acid, ornitric acid), base (e.g., potassium hydroxide), or salt (e.g., ammoniumsalt, such as a nitrate), as well any other suitable electrolyte knownin the art, such as a salt dissolved in an organic solvent (e.g.,ammonium salt dissolved in a glycol-based solution). Various otherelectrolytes are described in U.S. Pat. Nos. 5,369,547 and 6,594,140 toEvans, et al.

The desired ionic conductivity may be achieved by selecting ioniccompound(s) (e.g., acids, bases, salts, and so forth) within certainconcentration ranges. In one particular embodiment, salts of weakorganic acids may be effective in achieving the desired conductivity ofthe electrolyte. The cation of the salt may include monatomic cations,such as alkali metals (e.g., Li⁺, Na⁺, K⁺, Rb⁺, or Cs⁺), alkaline earthmetals (e.g., Be²⁺, Mg²⁺, Ca²⁺, Sr²⁺ or Ba²⁺), transition metals (e.g.,Ag⁺, Fe²⁺, Fe³⁺, etc.), as well as polyatomic cations, such as NH₄ ⁺.The monovalent ammonium (NH₄ ⁺), sodium (K⁺), and lithium (Li⁺) areparticularly suitable cations for use in the present invention. Theorganic acid used to form the anion of the salt may be “weak” in thesense that it typically has a first acid dissociation constant (pK_(a1))of about 0 to about 11, in some embodiments about 1 to about 10, and insome embodiments, from about 2 to about 10, determined at about 23° C.Any suitable weak organic acids may be used in the present invention,such as carboxylic acids, such as acrylic acid, methacrylic acid,malonic acid, succinic acid, salicylic acid, sulfosalicylic acid, adipicacid, maleic acid, malic acid, oleic acid, gallic acid, tartaric acid(e.g., dextotartaric acid, mesotartaric acid, etc.), citric acid, formicacid, acetic acid, glycolic acid, oxalic acid, propionic acid, phthalicacid, isophthalic acid, glutaric acid, gluconic acid, lactic acid,aspartic acid, glutaminic acid, itaconic acid, trifluoroacetic acid,barbituric acid, cinnamic acid, benzoic acid, 4-hydroxybenzoic acid,aminobenzoic acid, etc.; blends thereof, and so forth. Polyprotic acids(e.g., diprotic, triprotic, etc.) are particularly desirable for use informing the salt, such as adipic acid (pK_(a1) of 4.43 and pK_(a2) of5.41), α-tartaric acid (pK_(a1) of 2.98 and pK_(a2) of 4.34),meso-tartaric acid (pK_(a1) of 3.22 and pK_(a2) of 4.82), oxalic acid(pK_(a1) of 1.23 and pK_(a2) of 4.19), lactic acid (pK_(a1) of 3.13,pK_(a2) of 4.76, and pK_(a3) of 6.40), etc.

While the actual amounts may vary depending on the particular saltemployed, its solubility in the solvent(s) used in the electrolyte, andthe presence of other components, such weak organic acid salts aretypically present in the electrolyte in an amount of from about 0.1 toabout 25 wt. %, in some embodiments from about 0.2 to about 20 wt. %, insome embodiments from about 0.3 to about 15 wt. %, and in someembodiments, from about 0.5 to about 5 wt. %.

The electrolyte is typically aqueous in that it contains an aqueoussolvent, such as water (e.g., deionized water). For example, water(e.g., deionized water) may constitute from about 20 wt. % to about 95wt. %, in some embodiments from about 30 wt. % to about 90 wt. %, and insome embodiments, from about 40 wt. % to about 85 wt. % of theelectrolyte. A secondary solvent may also be employed to form a solventmixture. Suitable secondary solvents may include, for instance, glycols(e.g., ethylene glycol, propylene glycol, butylene glycol, triethyleneglycol, hexylene glycol, polyethylene glycols, ethoxydiglycol,dipropyleneglycol, etc.); glycol ethers (e.g., methyl glycol ether,ethyl glycol ether, isopropyl glycol ether, etc.); alcohols (e.g.,methanol, ethanol, n-propanol, iso-propanol, and butanol); ketones(e.g., acetone, methyl ethyl ketone, and methyl isobutyl ketone); esters(e.g., ethyl acetate, butyl acetate, diethylene glycol ether acetate,methoxypropyl acetate, ethylene carbonate, propylene carbonate, etc.);amides (e.g., dimethylformamide, dimethylacetamide,dimethylcaprylic/capric fatty acid amide and N-alkylpyrrolidones);sulfoxides or sulfones (e.g., dimethyl sulfoxide (DMSO) and sulfolane);and so forth. Such solvent mixtures typically contain water in an amountfrom about 40 wt. % to about 80 wt. %, in some embodiments from about 50wt. % to about 75 wt. %, and in some embodiments, from about 55 wt. % toabout 70 wt. % and secondary solvent(s) in an amount from about 20 wt. %to about 60 wt. %, in some embodiments from about 25 wt. % to about 50wt. %, and in some embodiments, from about 30 wt. % to about 45 wt. %.The secondary solvent(s) may, for example, constitute from about 5 wt. %to about 45 wt. %, in some embodiments from about 10 wt. % to about 40wt. %, and in some embodiments, from about 15 wt. % to about 35 wt. % ofthe electrolyte.

If desired, the electrolyte may be relatively neutral and have a pH offrom about 4.5 to about 8.0, in some embodiments from about 5.0 to about7.5, and in some embodiments, from about 5.5 to about 7.0. One or morepH adjusters (e.g., acids, bases, etc.) may be employed to help achievethe desired pH. In one embodiment, an acid is employed to lower the pHto the desired range. Suitable acids include, for instance, organicacids such as described above; inorganic acids, such as hydrochloricacid, nitric acid, sulfuric acid, phosphoric acid, polyphosphoric acid,boric acid, boronic acid, etc.; and mixtures thereof. Although the totalconcentration of pH adjusters may vary, they are typically present in anamount of from about 0.01 wt. % to about 10 wt. %, in some embodimentsfrom about 0.05 wt. % to about 5 wt. %, and in some embodiments, fromabout 0.1 wt. % to about 2 wt. % of the electrolyte.

The electrolyte may also contain other components that help improve theelectrical performance of the capacitor. For instance, a depolarizer maybe employed in the electrolyte to help inhibit the evolution of hydrogengas at the cathode of the electrolytic capacitor, which could otherwisecause the capacitor to bulge and eventually fail. When employed, thedepolarizer normally constitutes from about 1 to about 500 parts permillion (“ppm”), in some embodiments from about 10 to about 200 ppm, andin some embodiments, from about 20 to about 150 ppm of the electrolyte.Suitable depolarizers may include nitroaromatic compounds, such as2-nitrophenol, 3-nitrophenol, 4-nitrophenol, 2-nitrobenzonic acid,3-nitrobenzonic acid, 4-nitrobenzonic acid, 2-nitroace tophenone,3-nitroacetophenone, 4-nitroacetophenone, 2-nitroanisole,3-nitroanisole, 4-nitroanisole, 2-nitrobenzaldehyde,3-nitrobenzaldehyde, 4-nitrobenzaldehyde, 2-nitrobenzyl alcohol,3-nitrobenzyl alcohol, 4-nitrobenzyl alcohol, 2-nitrophthalic acid,3-nitrophthalic acid, 4-nitrophthalic acid, and so forth. Particularlysuitable nitroaromatic depolarizers for use in the present invention arenitrobenzoic acids, anhydrides or salts thereof, substituted with one ormore alkyl groups (e.g., methyl, ethyl, propyl, butyl, etc.). Specificexamples of such alkyl-substituted nitrobenzoic compounds include, forinstance, 2-methyl-3-nitrobenzoic acid; 2-methyl-6-nitrobenzoic acid;3-methyl-2-nitrobenzoic acid; 3-methyl-4-nitrobenzoic acid;3-methyl-6-nitrobenzoic acid; 4-methyl-3-nitrobenzoic acid; anhydridesor salts thereof; and so forth.

III. Cathode

A. Metal Substrate

The cathode typically contains a metal substrate, which may alsooptionally serve as a casing for the capacitor. The substrate may beformed from a variety of different metals, such as tantalum, niobium,aluminum, nickel, hafnium, titanium, copper, silver, steel (e.g.,stainless), alloys thereof, composites thereof (e.g., metal coated withelectrically conductive oxide), and so forth. The geometricconfiguration of the substrate may generally vary as is well known tothose skilled in the art, such as in the form of a foil, sheet, screen,container, can, etc. The metal substrate may form the all or a portionof casing for the capacitor, or it may simply be applied to the casing.Regardless, the substrate may have a variety of shapes, such asgenerally cylindrical, D-shaped, rectangular, triangular, prismatic,etc. If desired, a surface of the substrate may be roughened to increaseits surface area and increase the degree to which a material may be ableto adhere thereto. In one embodiment, for example, a surface of thesubstrate is chemically etched, such as by applying a solution of acorrosive substance (e.g., hydrochloric acid) to the surface. Mechanicalroughening may also be employed. For instance, a surface of thesubstrate may be abrasive blasted by propelling a stream of abrasivemedia (e.g., sand) against at least a portion of a surface thereof.

B. Conductive Coating

A conductive coating may also be disposed on a surface of the metalsubstrate (e.g., interior surface) to serve as an electrochemicallyactive material for the capacitor. Any number of layers may be employedin the coating. The materials employed in the coating may vary. Forexample, the conductive coating may contain a noble metal (e.g.,ruthenium, rhodium, palladium, silver, osmium, iridium, platinum, gold,etc.), an oxide (e.g., ruthenium oxide), carbonaceous materials,conductive polymers, etc. In one embodiment, for example, the coatingmay include conductive polymer(s) that are typically π-conjugated andhave electrical conductivity after oxidation or reduction. Examples ofsuch π-conjugated conductive polymers include, for instance,polyheterocycles (e.g., polypyrroles, polythiophenes, polyanilines,etc.), polyacetylenes, poly-p-phenylenes, polyphenolates, and so forth.Substituted polythiophenes are particularly suitable for use asconductive polymer in that they have particularly good mechanicalrobustness and electrical performance. In one particular embodiment, thesubstituted polythiophene has the following general structure:

wherein,

T is O or S;

D is an optionally substituted C₁ to C₅ alkylene radical (e.g.,methylene, ethylene, n-propylene, n-butylene, n-pentylene, etc.);

R₇ is a linear or branched, optionally substituted C₁ to C₁₈ alkylradical (e.g., methyl, ethyl, n- or iso-propyl, n-, iso-, sec- ortert-butyl, n-pentyl, 1-methylbutyl, 2-methylbutyl, 3-methylbutyl,1-ethylpropyl, 1,1-dimethylpropyl, 1,2-dimethylpropyl,2,2-dimethylpropyl, n-hexyl, n-heptyl, n-octyl, 2-ethylhexyl, n-nonyl,n-decyl, n-undecyl, n-dodecyl, n-tridecyl, n-tetradecyl, n-hexadecyl,n-octadecyl, etc.); optionally substituted C₅ to C₁₂ cycloalkyl radical(e.g., cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl, cyclononylcyclodecyl, etc.); optionally substituted C₆ to C₁₄ aryl radical (e.g.,phenyl, naphthyl, etc.); optionally substituted C₇ to C₁₈ aralkylradical (e.g., benzyl, o-, m-, p-tolyl, 2,3-, 2,4-, 2,5-, 2-6, 3-4-,3,5-xylyl, mesityl, etc.); optionally substituted C₁ to C₄ hydroxyalkylradical, or hydroxyl radical; and

q is an integer from 0 to 8, in some embodiments, from 0 to 2, and inone embodiment, 0; and

n is from 2 to 5,000, in some embodiments from 4 to 2,000, and in someembodiments, from 5 to 1,000. Example of substituents for the radicals“D” or “R₇” include, for instance, alkyl, cycloalkyl, aryl, aralkyl,alkoxy, halogen, ether, thioether, disulphide, sulfoxide, sulfone,sulfonate, amino, aldehyde, keto, carboxylic acid ester, carboxylicacid, carbonate, carboxylate, cyano, alkylsilane and alkoxysilanegroups, carboxylamide groups, and so forth.

Particularly suitable thiophene polymers are those in which “D” is anoptionally substituted C₂ to C₃ alkylene radical. For instance, thepolymer may be optionally substituted poly(3,4-ethylenedioxythiophene),which has the following general structure:

Methods for forming conductive polymers, such as described above, arewell known in the art. For instance, U.S. Pat. No. 6,987,663 to Merker,et al. describes various techniques for forming substitutedpolythiophenes from a monomeric precursor. The monomeric precursor may,for instance, have the following structure:

wherein,

T, D, R₇, and q are defined above. Particularly suitable thiophenemonomers are those in which “D” is an optionally substituted C₂ to C₃alkylene radical. For instance, optionally substituted3,4-alkylenedioxythiophenes may be employed that have the generalstructure:

wherein, R₇ and q are as defined above. In one particular embodiment,“q” is 0. One commercially suitable example of 3,4-ethylenedioxthiopheneis available from Heraeus Clevios under the designation Clevios™ M.Other suitable monomers are also described in U.S. Pat. No. 5,111,327 toBlohm, et al. and U.S. Pat. No. 6,635,729 to Groenendaal, et al.Derivatives of these monomers may also be employed that are, forexample, dimers or trimers of the above monomers. Higher molecularderivatives, i.e., tetramers, pentamers, etc. of the monomers aresuitable for use in the present invention. The derivatives may be madeup of identical or different monomer units and used in pure form and ina mixture with one another and/or with the monomers. Oxidized or reducedforms of these precursors may also be employed.

The thiophene monomers may be chemically polymerized in the presence ofan oxidative catalyst. The oxidative catalyst typically includes atransition metal cation, such as iron(III), copper(II), chromium(VI),cerium(IV), manganese(IV), manganese(VII), ruthenium(III) cations, etc.A dopant may also be employed to provide excess charge to the conductivepolymer and stabilize the conductivity of the polymer. The dopanttypically includes an inorganic or organic anion, such as an ion of asulfonic acid. In certain embodiments, the oxidative catalyst employedin the precursor solution has both a catalytic and doping functionalityin that it includes a cation (e.g., transition metal) and anion (e.g.,sulfonic acid). For example, the oxidative catalyst may be a transitionmetal salt that includes iron(III) cations, such as iron(III) halides(e.g., FeCl₃) or iron(III) salts of other inorganic acids, such asFe(ClO₄)₃ or Fe₂(SO₄)₃ and the iron(III) salts of organic acids andinorganic acids comprising organic radicals. Examples of iron (III)salts of inorganic acids with organic radicals include, for instance,iron(III) salts of sulfuric acid monoesters of C₁ to C₂₀ alkanols (e.g.,iron(III) salt of lauryl sulfate). Likewise, examples of iron(III) saltsof organic acids include, for instance, iron(III) salts of C₁ to C₂₀alkane sulfonic acids (e.g., methane, ethane, propane, butane, ordodecane sulfonic acid); iron (III) salts of aliphatic perfluorosulfonicacids (e.g., trifluoromethane sulfonic acid, perfluorobutane sulfonicacid, or perfluorooctane sulfonic acid); iron (III) salts of aliphaticC₁ to C₂₀ carboxylic acids (e.g., 2-ethylhexylcarboxylic acid); iron(III) salts of aliphatic perfluorocarboxylic acids (e.g.,trifluoroacetic acid or perfluorooctane acid); iron (III) salts ofaromatic sulfonic acids optionally substituted by C₁ to C₂₀ alkyl groups(e.g., benzene sulfonic acid, o-toluene sulfonic acid, p-toluenesulfonic acid, or dodecylbenzene sulfonic acid); iron (III) salts ofcycloalkane sulfonic acids (e.g., camphor sulfonic acid); and so forth.Mixtures of these above-mentioned iron(III) salts may also be used.Iron(III)-p-toluene sulfonate, iron(III)-o-toluene sulfonate, andmixtures thereof, are particularly suitable. One commercially suitableexample of iron(III)-p-toluene sulfonate is available from HeraeusClevios under the designation Clevios™ C.

Various methods may be utilized to form the conductive layer. In oneembodiment, the oxidative catalyst and monomer are applied, eithersequentially or together, such that the polymerization reaction occursin situ on the substrate. Suitable application techniques may includescreen-printing, dipping, electrophoretic coating, and spraying, may beused to form a conductive polymer coating. As an example, a monomer mayinitially be mixed with the oxidative catalyst to form a precursorsolution. Once the mixture is formed, it may be applied to the metalsubstrate and then allowed to polymerize so that the conductive layer isformed. Alternatively, the oxidative catalyst and monomer may be appliedsequentially. In one embodiment, for example, the oxidative catalyst isdissolved in an organic solvent (e.g., butanol) and then applied as adipping solution. The substrate may then be dried to remove the solventtherefrom. Thereafter, the substrate may be dipped into a solutioncontaining the monomer. Polymerization is typically performed attemperatures of from about −10° C. to about 250° C., and in someembodiments, from about 0° C. to about 200° C., depending on theoxidizing agent used and desired reaction time. Suitable polymerizationtechniques, such as described above, may be described in more detail inU.S. Publication No. 2008/232037 to Biler.

While chemical polymerization techniques may be employed in certainembodiments, it is often desired to minimize the use of oxidativecatalysts in the capacitor as such materials can often lead to theformation of iron radicals (e.g., Fe²⁺ or Fe³⁺ ions). These radicalscan, in turn, lead to dielectric degradation at the high voltages oftenemployed during the use of the wet capacitor. Thus, anodicelectrochemical polymerization techniques may be employed in certainembodiments to form the conductive polymer. Such techniques generallyemploy a colloidal suspension that is generally free of iron-basedoxidative catalysts. For instance, the colloidal suspension typicallycontains less than about 0.5 wt. %, in some embodiments, less than about0.1 wt. %, and in some embodiments, less than about 0.05 wt. % (e.g., 0wt. %) of such iron-based oxidative catalysts.

The colloidal suspension may be in the form of a macroemulsion,microemulsion, solution, etc. depending on the particular nature of thecomponents of the suspension. Regardless, the suspension generallycontains a solvent that serves as a continuous phase within which theprecursor monomer is dispersed. Any of a variety of different solventsmay be employed in the colloidal suspension, such as alcohols, glycols,water, etc. In one particular embodiment, the colloidal suspension isaqueous in nature. If desired, other additives may also be employed inthe suspension to facilitate polymerization, such as surfactants (e.g.,nonionic, anionic, or cationic surfactants), dopants (e.g., sulfonicacids), defoaming agents, and so forth. Solvents (e.g., water) mayconstitute from about 50 wt. % to about 99 wt. %, in some embodimentsfrom about 70 wt. % to about 98 wt. % and in some embodiments, fromabout 80 wt. % to about 95 wt. %. The remaining components of thecolloidal suspension (e.g., precursor monomers, surfactants, andsulfonic acids) may likewise constitute from about 1 wt. % to about 50wt. %, in some embodiments from about 2 wt. % to about 30 wt. % and insome embodiments, from about 5 wt. % to about 20 wt. % of the colloidalsuspension.

To apply the colloidal suspension, any of a variety of suitableapplication techniques may be employed, such as screen-printing,dipping, electrophoretic coating, spraying, etc. Regardless of how it isapplied, the monomer within the colloidal suspension may be anodicallyelectrochemically-polymerized to form the conductive polymer layer. Inone embodiment, for example, the metal substrate is dipped into a bathcontaining the colloidal suspension of the present invention. A pair ofelectrodes may be disposed within the bath for electrolysis. Oneelectrode may be connected to the positive terminal of a power sourceand also in contact with the metal substrate. The other electrode may beconnected to the negative terminal of the power source and an additionalinert metal. During operation, the power source supplies a current feedto the electrodes in the electrochemical cell, thereby inducingelectrolysis of the electrolyte and oxidative polymerization of themonomer in the colloidal suspension, or solution, onto the metalsubstrate. Anodic electrochemical polymerization is generally performedat ambient temperature to ensure that the colloidal suspension does notphase separate. For example, the colloidal suspension may be kept at atemperature of from about 15° C. to about 80° C., in some embodimentsfrom about 20° C. to about 75° C., and in some embodiments, from about25° C. to about 50° C. The amount of time in which the metal substrateis in contact with the colloidal suspension during anodicelectrochemical polymerization may vary. For example, the metalsubstrate may be dipped into such a solution for a period of timeranging from about 10 seconds to about 10 minutes.

Multiple polymerization steps may be repeated until the desiredthickness of the coating is achieved. In one embodiment, for example, achemically polymerized layer may be formed directly over the noble metallayer and an electrochemical polymerized layer may be disposedthereover, or vice versa. Regardless, the total target thickness of theconductive polymer layer(s) may generally vary depending on the desiredproperties of the capacitor. Typically, the resulting conductive polymerlayer(s) have a thickness of from about 0.2 micrometers (“μm”) to about50 μm, in some embodiments from about 0.5 μm to about 20 μm, and in someembodiments, from about 1 μm to about 5 μm. It should be understood thatthe thickness of the layers are not necessarily the same at alllocations on the substrate. Nevertheless, the average thickness on thesubstrate generally falls within the ranges noted above.

The particular manner in which the components are incorporated into thecapacitor is not critical and may be accomplished using a variety oftechniques. In most embodiments, however, the anode is positioned withina casing. Referring to FIGS. 1 and 4, for example, one embodiment of acapacitor 10 is shown that includes the anode 200 shown in FIGS. 2-3.Although only one anode is shown, it should be understood that multipleanodes (e.g., stack) may be employed as is described, for instance, inU.S. Pat. No. 7,483,260 to Ziamiak, et al. In the illustratedembodiment, the anode 200 may be positioned within a casing 12 made of afirst casing member 14 and a second casing member 16. The first casingmember 14 has a face wall 18 joined to a surrounding sidewall 20, whichextends to an edge 22. The second casing member 16 may likewise containa second face wall 24 having a surrounding edge 26. In the illustrateembodiment, the second casing member 16 is thus in the form of a platethat serves as a lid for the casing 10. The casing members 14 and 16 maybe hermetically sealed together by welding (e.g., laser welding) theedges 22 and 26 where they contact each other. The casing members 14and/or 16 may be analogous to the metal substrate described above suchthat a conductive polymer coating (not shown) may be deposited on theinterior surface thereof. Alternatively, a separate metal substrate maybe located adjacent to the casing member 14 and/or 16 and applied withthe conductive polymer coating.

Although not shown, one or more separators may be employed between theanode and cathode (e.g., between the anode 200 and the first casingmember 14, between the anode 200 and the second casing member 16, orbetween the anode and both casing members) that help insulate the anodeand conductive polymer-coated cathode from each other. Examples ofsuitable materials for this purpose include, for instance, porouspolymer materials (e.g., polypropylene, polyethylene, etc.), porousinorganic materials (e.g., fiberglass mats, porous glass paper, etc.),ion exchange resin materials, etc. Particular examples include ionicperfluoronated sulfonic acid polymer membranes (e.g., Nafion™ from theE.I. DuPont de Nemeours & Co.), sulphonated fluorocarbon polymermembranes, polybenzimidazole (PBI) membranes, and polyether ether ketone(PEEK) membranes. Although preventing direct contact between the anodeand cathode, the separator permits ionic current flow of the electrolyteto the electrodes.

A feedthrough 30 (FIG. 1) may also be employed that electricallyinsulates the anode wire 200 from the casing 12. The feedthrough 30extends from within the casing 12 to the outside thereof. A hole 34 maybe provided in the surrounding sidewall 20 of the casing member 14 intowhich the feedthrough 30. The feedthrough 30 may, for example, be aglass-to-metal seal (“GTMS”) that contains a ferrule (not shown) with aninternal cylindrical bore of a constant inside diameter. An insulativeglass can thus provide a hermetic seal between the bore and the anodewire 200 passing therethrough. After assembly and sealing (e.g.,welding), the electrolyte may optionally be introduced into the casingthrough a fill-port. Filling may be accomplished by placing thecapacitor in a vacuum chamber so that the fill-port extends into areservoir of the electrolyte. When the chamber is evacuated, pressure isreduced inside the capacitor. When the vacuum is released, pressureinside the capacitor re-equilibrates, and the electrolyte is drawnthrough the fill-port into the capacitor.

Regardless of its particular configuration, the capacitor of the presentinvention may exhibit excellent electrical properties. For example, thecapacitor may exhibit a high volumetric efficiency, such as from about50,000 μF*V/cm³ to about 300,000 μF*V/cm³, in some embodiments fromabout 60,000 μF*V/cm³ to about 200,000 μF*V/cm³, and in someembodiments, from about 80,000 μF*V/cm³ to about 150,000 μF*V/cm³,determined at a frequency of 120 Hz and at room temperature (e.g., 25°C.). Volumetric efficiency is determined by multiplying the formationvoltage of a part by its capacitance, and then dividing by the productby the volume of the part. For example, a formation voltage may be 175volts for a part having a capacitance of 520 μF, which results in aproduct of 91,000 μF*V. If the part occupies a volume of about 0.8 cm³,this results in a volumetric efficiency of about 113,750 μF*V/cm³.

The capacitor may also exhibit a high energy density that enables itsuitable for use in high pulse applications. Energy density is generallydetermined according to the equation E=½*CV², where C is the capacitancein farads (F) and V is the working voltage of capacitor in volts (V).The capacitance may, for instance, be measured using a capacitance meter(e.g., Keithley 3330 Precision LCZ meter with Kelvin Leads, 2 volts biasand 1 volt signal) at operating frequencies of from 10 to 120 Hz (e.g.,120 Hz) and a temperature of 25° C. For example, the capacitor mayexhibit an energy density of about 2.0 joules per cubic centimeter(J/cm³) or more, in some embodiments about 3.0 J/cm³, in someembodiments from about 3.5 J/cm³ to about 10.0 J/cm³, and in someembodiments, from about 4.0 to about 8.0 J/cm³. The capacitance maylikewise be about 1 milliFarad per square centimeter (“mF/cm²”) or more,in some embodiments about 2 mF/cm² or more, in some embodiments fromabout 5 to about 50 mF/cm², and in some embodiments, from about 8 toabout 20 mF/cm². The capacitor may also exhibit a relatively high“breakdown voltage” (voltage at which the capacitor fails), such asabout 180 volts or more, in some embodiments about 200 volts or more,and in some embodiments, from about 210 volts to about 260 volts.

The equivalent series resistance (“ESR”)—the extent that the capacitoracts like a resistor when charging and discharging in an electroniccircuit—may also be less than about 15,000 milliohms, in someembodiments less than about 10,000 milliohms, in some embodiments lessthan about 5,000 milliohms, and in some embodiments, from about 1 toabout 4,500 milliohms, measured with a 2-volt bias and 1-volt signal ata frequency of 120 Hz. In addition, the leakage current, which generallyrefers to the current flowing from one conductor to an adjacentconductor through an insulator, can be maintained at relatively lowlevels. For example, the numerical value of the normalized leakagecurrent of a capacitor of the present invention is, in some embodiments,less than about 1 μA/μF*V, in some embodiments less than about 0.5μA/μF*V, and in some embodiments, less than about 0.1 μA/μF*V, where μAis microamps and μF*V is the product of the capacitance and the ratedvoltage. Leakage current may be measured using a leakage test meter(e.g., MC 190 Leakage test, Mantracourt Electronics LTD, UK) at atemperature of 25° C. and at a certain rated voltage after a chargingtime of from about 60 to about 300 seconds. Such ESR and normalizedleakage current values may even be maintained after aging for asubstantial amount of time at high temperatures. For example, the valuesmay be maintained for about 100 hours or more, in some embodiments fromabout 300 hours to about 2500 hours, and in some embodiments, from about400 hours to about 1500 hours (e.g., 500 hours, 600 hours, 700 hours,800 hours, 900 hours, 1000 hours, 1100 hours, or 1200 hours) attemperatures ranging from about 100° C. to about 250° C., and, in someembodiments from about 100° C. to about 200° C. (e.g., 100° C., 125° C.,150° C., 175° C., or 200° C.).

The electrolytic capacitor of the present invention may be used invarious applications, including but not limited to implantable medicaldevices, such as implantable defibrillators, pacemakers, cardioverters,neural stimulators, drug administering devices, etc. In one embodiment,for example, the capacitor may be employed in an implantable medicaldevice configured to provide a therapeutic high voltage (e.g., betweenapproximately 500 volts and approximately 850 volts, or, desirably,between approximately 600 Volts and approximately 900 volts) treatmentfor a patient. The device may contain a container or housing that ishermetically sealed and biologically inert. One or more leads areelectrically coupled between the device and the patient's heart via avein. Cardiac electrodes are provided to sense cardiac activity and/orprovide a voltage to the heart. At least a portion of the leads (e.g.,an end portion of the leads) may be provided adjacent or in contact withone or more of a ventricle and an atrium of the heart. The device mayalso contain a capacitor bank that typically contains two or morecapacitors connected in series and coupled to a battery that is internalor external to the device and supplies energy to the capacitor bank. Duein part to high conductivity, the capacitor of the present invention canachieve excellent electrical properties and thus be suitable for use inthe capacitor bank of the implantable medical device.

The present invention may be better understood by reference to thefollowing example.

Test Procedures

Capacitance (“CAP”), equivalent series resistance (“ESR”) and leakagecurrent (“DCL”) were tested in an aqueous neutral electrolyte at atemperature of 37° C.±0.5° C.

Capacitance (“CAP”)

Capacitance may be measured using a Keithley 3330 Precision LCZ meterwith Kelvin Leads with 2.2 volt DC bias and a 0.5 volt peak to peaksinusoidal signal. The operating frequency may be 120 Hz.

Equivalent Series Resistance (“ESR”)

Equivalence series resistance may be measured using a Keithley 3330Precision LCZ meter with Kelvin Leads 2.2 volt DC bias and a 0.5 voltpeak to peak sinusoidal signal. The operating frequency may be 120 Hz.

Leakage Current (“DCL”)

Leakage current may be determined by charging to 250V for 300 secondswithout any resistor in series.

Example

Anodes were formed from a nodular, magnesium-reduced tantalum powder(H.C. Starck) and flake tantalum powder (Global Advanced Metals).Samples of each powder type were pressed to 5.3 g/cm³ density using 4%of stearic acid lubricant. After delubrication, samples of each powderwere then vacuum sintered at 1400° C. for 10 minutes in a hangingcrucible. Upon sintering, the pellets were anodized in a solutioncontaining 50% glycol/water with phosphoric acid at a temperature of 85°C. and a conductivity of 1 mS/cm. The formation current density was 45mA/g for each sample. Anodization voltage of 220 volts was tested. Theresulting anodes had a D-shape in which the length was about 32millimeters, the width was about 23 millimeters, and the thickness wasabout 2 millimeters. The anodes were then joined together with twocathodes prepared from Pd/PEDT coated titanium sheets (0.1 mm thick)separated with two plastic meshes (0.2 mm thick). The resultingcapacitors were then tested as described above. The results are setforth below.

Tantalum CAP ESR DCL CV/cc CV/g Powder [μF] [Ohm] [μA/g] [μF/cm³] [μF/g]NODULAR 465 1.42 33 93238 15781 FLAKE 360 1.40 116 71978 11800

As shown, the nodular powder was able to achieve a comparatively highcapacitance and lower DCL. Using the capacitance values obtained andassuming an operating voltage, energy density (E=0.5*CV²) indicated thenodular powder also had a significantly higher energy density.

These and other modifications and variations of the present inventionmay be practiced by those of ordinary skill in the art, withoutdeparting from the spirit and scope of the present invention. Inaddition, it should be understood that aspects of the variousembodiments may be interchanged both in whole or in part. Furthermore,those of ordinary skill in the art will appreciate that the foregoingdescription is by way of example only, and is not intended to limit theinvention so further described in such appended claims.

What is claimed is:
 1. A wet electrolytic capacitor comprising: a planaranode that comprises an anodically oxidized pellet formed from a pressedand sintered valve metal powder, wherein the valve metal powder isformed by reacting an oxide of a valve metal compound with a reducingagent that contains a metal having an oxidation state of 2 or more; acathode that comprises a metal substrate coated with a conductivecoating; and a fluidic working electrolyte in communication with theanode and the cathode.
 2. The capacitor of claim 1, wherein the metal isan alkaline earth metal, aluminum, or a combination thereof.
 3. Thecapacitor of claim 2, wherein the metal is magnesium.
 4. The capacitorof claim 1, wherein the oxide is tantalum pentoxide.
 5. The capacitor ofclaim 1, wherein the valve metal powder includes tantalum.
 6. Thecapacitor of claim 1, wherein the valve metal powder contains particleshaving an aspect ratio of about 4 or less.
 7. The capacitor of claim 6,wherein the particles are nodular or angular particles.
 8. The capacitorof claim 1, wherein the valve metal powder contains particles having amedian size of from about 5 to about 1000 nanometers.
 9. The capacitorof claim 1, wherein the powder has a specific surface area of about 1square meter per gram or more.
 10. The capacitor of claim 1, wherein thepowder has a specific charge of about 15,000 μF*V/g or more.
 11. Thecapacitor of claim 1, wherein the powder has no more than about 50 ppmof alkali metals.
 12. The capacitor of claim 1, wherein the anode has athickness of about 5 millimeters or less.
 13. The capacitor of claim 1,wherein a leadwire extends from the planar anode.
 14. The capacitor ofclaim 1, wherein the anode has a D-shape.
 15. The capacitor of claim 1,wherein the metal substrate includes titanium or stainless steel. 16.The capacitor of claim 1, wherein the conductive coating includes asubstituted polythiophene.
 17. The capacitor of claim 1, wherein theelectrolyte has a pH of from about 5.0 to about 7.5.
 18. The capacitorof claim 1, wherein a separator is positioned between the anode andcathode.
 19. The capacitor of claim 1, wherein the capacitor contains acasing that contains a first casing member and a second casing memberbetween which the anode and the fluid working electrolyte are disposed,wherein the metal substrate forms at least a portion of the first casingmember, the second casing member, or both.
 20. The capacitor of claim19, wherein the first casing member contains a face wall and asurrounding sidewall that extends to an edge, and further wherein thesecond casing member is in the form of a lid that is sealed to the edgeof the sidewall.
 21. An implantable medical device comprising thecapacitor of claim
 1. 22. A wet electrolytic capacitor comprising: aplanar anode that comprises an anodically oxidized pellet formed from apressed and sintered tantalum powder, wherein the powder is nodular orangular and has a specific charge of about 15,000 μF*V/g or more; acathode that comprises a metal substrate coated with a conductivecoating; and a fluidic working electrolyte in communication with theanode and the cathode.
 23. The capacitor of claim 22, wherein thetantalum powder is formed by reacting an oxide of tantalum with areducing agent that contains magnesium, strontium, barium, cesium,calcium, aluminum, or a combination thereof.
 24. The capacitor of claim22, wherein the powder has a specific surface area of from about 4 toabout 30 meters squared per gram.
 25. The capacitor of claim 22, whereinthe powder has no more than about 50 ppm of alkali metals.
 26. A methodfor forming a wet electrolytic capacitor, the method comprising:pressing a tantalum powder into the form of a pellet, wherein the powderis formed by reacting tantalum pentoxide with a reducing agent thatcontains magnesium, calcium, strontium, barium, cesium, aluminum, or acombination thereof; sintering the pellet; anodically oxidizing thesintered pellet to form a dielectric layer that overlies the anode; andpositioning the anode and a fluidic working electrolyte within a casing.